There are many and pervasive needs for treating gases (to include any substances therein), particularly for environmental purposes. Typically, the objective is to render such a gas physically clean and safe for plant and animal life by making it non-toxic and non-flammable. The physical cleaning may include removal of chemical and/or particulate matter, while toxicity and flammability may involve chemical reactions and/or dilution with safe substances. A popular treating expedient utilizes a liquid such as water to produce a waste gas and a waste solution which can be further treated by commonly available processes such as in waste treatment plants. When a gaseous waste requires both chemical reaction and physical cleaning, the treatment is a challenging task.
One such task is to treat gases exhausted from reactors used to grow layers of mono or polycrystalline material on wafers in semiconductor work. For example, the exhaust from a typical silicon reactor includes hydrogen, hydrogen chloride, various chlorosilanes and trace gases. The hydrogen can be very flammable in air, the hydrogen chloride is toxic and corrosive and the chlorosilanes may be flammable, toxic and corrosive. Conveniently, such gases react with or are absorbed by water to form soluble and/or deposition products and free hydrogen. A problem is to deliver the gases to a water treating process safely and efficiently.
The chlorosilanes tend to polymerize even in the absence of air to form fine particulates which may be gas-borne. They also polymerize in the presence of moisture to form stubborn deposits in delivery piping. Such deposits cause clogging and include substances which burn violently in air, especially during clean-out scraping.
Air should be excluded from the delivery piping to avoid hydrogen fires or a sufficient quantity should be utilized to safely dilute the hydrogen. Moisture should be excluded to avoid deposition reactions and consequent clogging. A problem is that air gets into conventional gas treating equipment. Subsequently, the air back-diffuses into the delivery piping carrying with it water mists which aggravate the delivery problem. One solution has been to continually purge the delivery piping with an inert gas such as nitrogen. A problem is that so much nitrogen is required to totally exclude the air and the mists that the resultant volume of nitrogen and waste gases causes excessive enlargement of the subsequent liquid treating equipment.
The equipment for treating gases with a liquid includes wet scrubbers for capture of particulate matter and absorption equipment for the dissolution and/or reaction of gases. Fortunately, the equipment for particulate removal with a liquid is similar and often identical to absorption equipment. Such equipment includes impingement apparatus, packed and plate towers, venturi devices, bubblers, spray apparatus, pumps and fans. The fans are generally used to dilute troublesome gas such as hydrogen so it represents less than 4% by volume of the combined air and hydrogen (the lower explosive limit of hydrogen in air).
Nearly all the above equipment has been tried in the past to treat gases from silicon reactors. For example, tower scrubbers packed with high surface media for developing water films have been tried and the media quickly clogged with deposition products. Horizontal gas flowing scrubbers having cross sprays were utilized with better but limited success. Multiple channel spray chambers were tried wherein the gas moved alternately with (co-current) and against (counter-current) the sprays with poor clogging experience. Scrubbers of both vertical and horizontal design were sometimes utilized with fans to draw ambient air to dilute the hydrogen for fire safety and to discourage back diffusion of mists into delivery piping. Unfortunately, such air flow added oxygen to accelerate deposition reactions, increased the overall size of expensive equipment and increased the volume of water and energy required to do the scrubbing. Also, when alternating spray chambers were employed, they were open to the atmosphere whereby air and water mists back-diffused into delivery piping even when nitrogen was utilized to move the reactor gases.
Accordingly, it is desirable to develop new and improved expedients for treating gases with liquids. Such expedients should include the ability to at least partially treat for toxicity, flammability and physical cleaning. The end products of such treatment should include gases and solutions which may be treated by available, common processes, e.g., air dilution and in conventional waste treatment plants. It is further desirable to include expedients for protecting gas delivery piping from undesirable back-diffusion of substances such as air and moisture. Hopefully, the size and cost of treating apparatus and the cost of operating the same may be minimized by improved expedients. One such expedient should facilitate diluting troublesome gases at the end and not the beginning of treatment so dilution gases need not be treated. Another expedient should facilitate spraying to obtain efficient treatment and adequate flushing of fast reacting deposits from apparatus. Still another expedient should facilitate advancing gases and reaction products through passageways that are self-cleaning or at least slow clogging.